The present invention relates to in-circuit testing of specialized graphics-processing chips.
Background: 3D Computer Graphics
One of the driving features in the performance of most single-user computers is computer graphics. This is particularly important in computer games and workstations, but is generally very important across the personal computer market.
For some years the most critical area of graphics development has been in three-dimensional (xe2x80x9c3Dxe2x80x9d) graphics. The peculiar demands of 3D graphics are driven by the need to present a realistic view, on a computer monitor, of a three-dimensional scene. The pattern written onto the two-dimensional screen must therefore be derived from the three-dimensional geometries in such a way that the user can easily xe2x80x9cseexe2x80x9d the three-dimensional scene (as if the screen were merely a window into a real three-dimensional scene). This requires extensive computation to obtain the correct image for display, taking account of surface textures, lighting, shadowing, and other characteristics.
The starting point (for the aspects of computer graphics considered in the present application) is a three-dimensional scene, with specified viewpoint and lighting (etc.). The elements of a 3D scene are normally defined by sets of polygons (typically triangles), each having attributes such as color, reflectivity, and spatial location. (For example, a walking human, at a given instant, might be translated into a few hundred triangles which map out the surface of the human""s body.) Textures are xe2x80x9cappliedxe2x80x9d onto the polygons, to provide detail in the scene. (For example, a flat carpeted floor will look far more realistic if a simple repeating texture pattern is applied onto it.) Designers use specialized modelling software tools, such as 3D Studio, to build textured polygonal models.
The 3D graphics pipeline consists of two major stages, or subsystems, referred to as geometry and rendering. The geometry stage is responsible for managing all polygon activities and for converting three-dimensional spatial data into a two-dimensional representation of the viewed scene, with properly-transformed polygons. The polygons in the three-dimensional scene, with their applied textures, must then be transformed to obtain their correct appearance from the viewpoint of the moment; this transformation requires calculation of lighting (and apparent brightness), foreshortening, obstruction, etc.
However, even after these transformations and extensive calculations have been done, there is still a large amount of data manipulation to be done: the correct values for EACH PIXEL of the transformed polygons must be derived from the two-dimensional representation. (This requires not only interpolation of pixel values within a polygon, but also correct application of properly oriented texture maps.) The rendering stage is responsible for these activities: it xe2x80x9crendersxe2x80x9d the two-dimensional data from the geometry stage to produce correct values for all pixels of each frame of the image sequence.
The most challenging 3D graphics applications are dynamic rather than static. In addition to changing objects in the scene, many applications also seek to convey an illusion of movement by changing the scene in response to the user""s input. Whenever a change in the orientation or position of the camera is desired, every object in a scene must be recalculated relative to the new view. As can be imagined, a fast-paced game needing to maintain a high frame rate will require many calculations and many memory accesses.
FIG. 2 shows a high-level overview of the processes performed in the overall 3D graphics pipeline. However, this is a very general overview, which ignores the crucial issues of what hardware performs which operations.
Background: Scan Chains
Scan chains are a standard part of the manufacturing process of most chips and give the chip tester a way of stimulating, and getting results from, a chip under test. Typically 10-20% of the area of the die is devoted to supporting this test methodology, and it is only used while the chip is on the tester and in a special test mode.
In-Circuit Test Using Scan Chains
A specialized processing chip (e.g. a graphics accelerator) in which the host interface provides access to the diagnostic registers in most of the complex logic on the chip, except for the host interface itself. This advantageously permits direct access to register contents for performance analysis as well as fault diagnosis.